The key was looking back at old bones. "If we ignore the fossil record and limit our studies to living sloths, as previous studies have done, there's a good chance that we'll miss out on the real story and maybe underestimate the extraordinarily complex evolution that produced the species that inhabit our world," Goswami says.

Though the ancient sloth lineages quickly grew into giants, today's sloths are having a tough time keeping up with the pace of their habitat destruction. And the sloths currently hanging out in Central and South America can't rely on size. Instead, many sloth species - like the threatened maned three-toed sloth - need our help.